Buried Submarine Pipelines Research Articles

Numerical Study on the Migration of Leaked Oil in Submarine Sediment Caused by Pipeline Leakage Using Two-phase Flow Model

Oil leakage from submarine pipelines can cause severe environmental problems and economic losses. In this study, a two-phase immiscible flow model in porous media is proposed to simulate the migration of leaked oil caused by the leakage of a buried submarine pipeline in sandy sediment. The relationship between saturation, pore pressure, and relative permeability of different fluids in porous media is considered in present model, which is distinguished from previous studies. The spilled oil spreads radially from the leakage point into surrounding sediment driven by the leakage velocity. Moreover, it continues to migrate in sediment under the capillary pressure and gravity. Interestingly, the effect of wave-induced transient response on the ultimate migration of spilled oil in a sandy sediment is negligible. However, increasing sediment porosity can greatly enhance the upward migration of leaked oil based on systematically parametric analysis. Meanwhile, the migration rate of leaked oil decreases with increasing oil density or viscosity. The oil-contaminated area significantly expands with the increase of leakage rate. Fitting formulas between dimensionless time required for the leaked oil to reach the overlying seawater and non-dimensional influencing parameters are well established, which can provide useful guidelines for similar disasters.

The uplift resistance of submarine pipelines buried in medium dense sand

Buried submarine pipelines need to be operated under high temperature and high pressure, and ensuring sufficient buried depth is a necessary factor for safe operation. Based on the plane strain assumption, the purpose of this study is to study the evolutionary mechanism of pipe-soil interaction under the monotonic uplift of buried submarine pipelines through model tests and the Discrete Element Method (DEM) numerical simulation. The results show that the uplift resistance is affected by the embedment ratio and the pipe diameter; during the pipeline uplift process, the development of the uplift resistance is divided into three stages: the development stage, the softening stage, and the residual stabilization stage. The displacement at the peak uplift resistance increases with the increase of the embedment ratio, and the value is about 0.5–1.5%H, and the sand particle size has a great influence on the displacement of the peak uplift resistance and the post-peak softening behavior. According to the comparison of the test results, the prediction model of the load-displacement response of pipe-soil interaction is proposed.

Investigations on the Effectiveness of Protection Methods for a Submarine Pipeline Exposed to the Impact of a Falling Anchor

The occurrence of a buried submarine pipeline crossing a channel becoming damaged by the impact of a falling anchor is becoming more common. It is important to analyze the dynamic response of pipelines exposed to such impact and develop effective protection methods to ensure the safe operation of the pipelines exposed to the impact of falling anchors. In this study, different protection methods, including pure rock, concrete mattress + rock, concrete mattress + rock + rubber pad, and compound flexible pad + rock, are physically tested. The strains at the impacting point and along the pipeline were measured with the fiber Bragg grating (FBG) sensors. The effectiveness of the protection methods is analyzed based on the maximum strain and its affected length on the pipeline. Then, a theoretical model is established to analyze the deformation and strain of a pipeline. Through curve-fitting the experimental results, the bearing capacity coefficients for different protection methods are determined. The protection method of compound flexible pad + rock has the best performance to protect the pipeline from the impact of a falling anchor.

Wave force reduction due to burial of submarine pipeline: A probability based analysis and comparison with DNVGL-RP-F109

Horizontal and vertical wave forces were assessed on submarine pipeline for different burial depths in soils of four different hydraulic conductivity in random waves of wide range of wave height and periods. Probability analysis is carried out for 99.99% and 98% probability of non-exceedance of vertical and horizontal wave forces on the pipeline. The force reduction due to burial of the pipelines are compared with the recommendations by DNVGL-RP-F109. The effect of changing the wave height, wave period and hydraulic conductivity on the wave forces and its change due to different burial depths are revealed. The wave force values and the quantitative differences due to 99.99% and 98% probability of non-exceedance can be used for risk-based design of buried pipelines. The recommended value of wave force reduction factor for different relative burial depth of the pipeline by DNVGL-RP-F109 is much smaller than the value of wave force reduction factor from the present study. Strong scope exists to revise this code for the betterment of submarine pipeline design engineers. The knowhow revealed in this study will be useful for optimal design of buried submarine pipelines.

Seismic dynamics of a pipeline shallowly buried in loosely deposited seabed foundation

It is widely known that submarine pipelines constructed in high seismic intensity zones are vulnerable due to soil liquefaction under the attack of seismic waves. It is meaningful for engineers to get insight into the seismic dynamic response mechanism of submarine pipelines, which mostly are buried in loosely deposited seabed floors in practical engineering. In this study, taking the integrated numerical model FSSI-CAS 2D as the platform, the seismic dynamic response of a shallowly buried submarine pipeline in a loosely deposited seabed floor is investigated. It is indicated by the numerical results that a shallowly buried submarine pipeline in loose seabed floor intensively responds to input seismic wave. Considerable vibration in horizontal and floatation in vertical occur for the pipeline due to the soil softening in the surrounding soil, as well as the soil liquefaction in the zone away from the pipeline. The buoyancy applied on the outer wall of the pipeline caused by the pore pressure accumulation makes a significant contribution to the upward floatation of the pipeline. It is found that the seismic wave-induced residual liquefaction firstly initiates in the medium depth of seabed floor due to the relatively great permeability of the loose seabed soil, and then develops toward to the upper and lower seabed synchronously. Because of the intensive interaction between the pipeline and its surrounding seabed soil, liquefaction has not occurred in the zone at the right and left-hand side of, as well as over the pipeline. Based on comparative analysis, it is found the magnitude of pipeline floatation if natural gas is transported is much greater than that if crude oil is transported. Finally, it is observed that considerable subsidence at the magnitude order of 1 m occurs for the pipeline in the post-reconsolidation process after seismic wave attacking. It is indicated by the analysis presented in this study that the integrated model FSSI-CAS 2D would be a trustworthy platform to study the seismic dynamic response of marine structures.

A mechanical model to determine upheaval buckling of buried submarine pipelines

Thermal loads in submarine pipelines generate an axial compressive load that can force the pipeline to buckle, leading to failure if these loads are not considered in the design. Buried pipes are constraint to displacements in all directions, which leads to a high compressive load in the longitudinal axis and makes the pipes more vulnerable to buckling. If buried pipes under thermal loads do not buckle, a high-stresses state takes place when it is combined with high-pressure conditions. In this work, a simple mechanical model to determine the axial buckling load of a buried pipeline is proposed. The model is based on a simply supported beam subjected to a distributed transverse load representing the soil uplift resistance obtained from a referenced model, and an axial compressive load that represents the effective axial force and is computed according to the DNV-RP-F110. Additionally, the pipe–soil system is analyzed through a non-linear finite element model to compare the results with the analytical solution. The proposed simple mechanical model can capture the upheaval buckling behavior and provides results that are consistent with the numerical analysis, specifically for the two main parameters evaluated, namely, the initial pipe curvature and the magnitude of the transverse load.

Dynamics of a pipeline buried in loosely deposited seabed to nonlinear wave [formula omitted] current

Submarine pipeline is a type of important infrastructure in petroleum industry, used for transporting crude oil or natural gas. Understanding of the dynamics characteristics under hydrodynamic loading is crucial for engineers when assessing the stability of offshore pipelines in their designed service period. In this study, taking the integrated numerical model FSSI-CAS 2D as the tool, the nonlinear ocean wave & current-induced dynamics of a shallowly buried submarine steel pipeline and its surrounding loosely deposited seabed soil is numerically investigated. The excellent soil model Pastor-Zienkiewicz-Mark III (PZIII) is adopted to describe the complicated mechanical behaviour of loose seabed soil under cyclic loading. Computational results indicate that the shallowly buried submarine pipeline eventually significantly float up, and extrude the seabed soil over the pipeline driven by the gradually increased buoyancy on the pipeline caused by the accumulation of pore pressure around the pipeline, making the seabed over the pipeline hunch significantly. As a result, considerable deformation occurs in the seabed soil surrounding the pipeline. Two effective stress-based criterion are proposed to judge the occurrence of soil liquefaction. Adopting the two criterion. it is found that the surrounding seabed soil around the pipeline does not become liquefied; only stiffness softening has occurred in it. However, the soil in the upper seabed with shallow depth away from the pipeline becomes liquefied with a liquefaction depth 1.2–1.5 m. Comparative analysis indicates that the pipeline transporting natural gas floats upward with a much greater displacement than that if crude oil is transported. The computational results show that the integrated mode FSSI-CAS 2D has successfully and subtly captured a series of nonlinear physical phenomena of the intensive interaction between pipeline and its surrounding seabed soil. Finally, it is indicated that the integrated model FSSI-CAS 2D has an advantage to investigate the complicated interaction between fluid-structure-seabed foundation.

Numerical finite element modelling of soil resistance against upheaval buckling of buried submarine pipelines

Submarine high-temperature and/or high-pressure (HT/HP) pipelines used to transport oil and gas in different areas of the world are susceptible to global buckling. If the pipelines are simply laid on the seabed, buckling will likely occur in the horizontal plane, while if buried in a trench, buckling will probably arise in the vertical direction. The latter buckling mode may lead to catastrophic pipeline failure potentially associated with massive oil leakage and severe environmental damage. Concerns about upheaval buckling have motivated a number of researchers to investigate the uplift resistance of buried pipelines. The objective of the work presented in this paper is to evaluate the effects of various parameters on the resistance to upheaval buckling of buried submarine pipelines. A numerical approach was adopted to build representative and reliable 3D models of 100 m long embedded pipelines, using the specialized finite element software ABAQUS. Starting with a baseline model of a pipe buried in a medium dense sand with fines, the effects of pertinent parameters on the uplift resistance were explored for two scenarios: (1) uplift of the full pipeline length, which in essence captures the plane strain 2D response and (2) uplift of a central middle length of 20 m. Specifically, the effects of the apparent cohesion of the soil due to the presence of fines, pipeline diameter, embedment depth and diameter to wall thickness ratio were investigated. The uplift forces obtained were normalized in reference to actual pullout and/or effective pipe lengths. The results indicated that the normalized uplift forces obtained in the 3D analyses at high pipeline displacements were in close agreement with those yielded by plane strain models. The model analyses indicated that the contribution of soil apparent cohesion to the uplift resistance was significant. The work indicated that as the level of confinement increases (due to an increase in embedment depth and/or pipeline diameter) the uplift resistance increased, whereas the relative contribution of soil cohesion to the resistance decreases. The paper includes a comparison of the FE results obtained in this study with the available predictive/analytical methods, along with a discussion of which best captured the failure mode and resistance to uplift values of the numerical model.

Seepage Force on a Buried Submarine Pipeline Induced by a Solitary Wave

In this study, a finite element method was used to establish a two-dimensional numerical model to solve the problem of the Biot equation describing the poroelastic seabed, and to analyze the seepage force on a buried submarine pipeline under the propagation of a solitary wave. The model provides a solution to the displacement of the poroelastic seabed and the variation of the pore-water pressure. By means of numerical simulation, the effects of Young’s modulus and permeability coefficient of the soil on the pore-water pressure and seepage force are discussed. In the simulation of solitary waves passing through fully buried submarine pipelines, numerical results indicate that the smaller the permeability coefficient in dense sandy bed the greater the vertical force acting on the pipeline, and the smaller the permeability coefficient in loose sand bed the smaller the vertical force acting on the pipeline. In general, when the permeability coefficient is large, the smaller the Young’s modulus the more obvious the influence of the vertical force on the pipeline, and when the permeability coefficient is small, the larger the Young’s modulus the more obvious the influence of the vertical force on the pipeline.

Undrained bearing capacity of skids/pedrails during trenching for buried submarine pipelines

The paper investigates the undrained bearing capacity of an underwater trencher used for installation of buried submarine pipelines. The underlying soil is assumed to have an undrained strength that increases linearly with depth. The paper presents charts for the bearing capacity of these systems as a function of trench geometry, loading distance (i.e., the distance between the trencher skid/pedrail and the crest of trench wall), surface strength and the gradient of strength increasing with depth. These charts can guide engineers in the level of buoyancy needed for safe trenching operations. All calculations are performed using finite element limit analysis (FELA) and the parametric studies reveal five principal failure modes. At last, the paper includes an engineering example to demonstrate the use of the charts for practice.

Undrained stability analysis of trenches for buried submarine pipelines

The stability of trenches for buried submarine pipelines (TBSPs) during excavation and/or prior to backfilling has not received enough attention in the literature. In this study, the undrained stability of TBSPs in horizontal and inclined seabeds with shear strengths increasing linearly with depth is investigated using the lower and upper bound finite element limit analysis (FELA). The surcharge due to excavated soils and trenching machines is reasonably considered. Extensive parametric studies are performed on the trench slope angle β, normalized width of trenching machine L/H, dimensionless strength gradient Hk/su0 and the volume ratio R (for inclined seabed only) of the excavated soil stacked on the upside and downside of trenches. The actual results are accurately bracketed by the computed upper and lower bound solutions. For the trench with horizontal seabeds, the maximum stability can be obtained under β = 70°–80°. For inclined seabeds, the global stability of TBSPs roughly reaches peak value for different combinations of L/H and β when R = 0.15–0.3.

An Effective Method for Submarine Buried Pipeline Detection via Multi-Sensor Data Fusion

Submarine pipelines are important resource delivery devices between land and ocean. For safety reasons, pipelines are often embedded beneath the seabed at a certain depth, to reduce the risk of direct damage to the pipeline. In the past, various kinds of detection equipment have been used for pipeline inspection, to ensure the normal operation of pipelines in practical applications. Acoustic detection technology is the dominant method to monitor buried submarine pipelines. Extracting and integrating the information in acoustic images, such as the route and burial depth, can help to monitor the status of a pipeline. However, most of the existing methods are based on limited parameters, and they cannot be used to precisely detect and locate a submarine pipeline under complex conditions. In this study, a multi-sensor surveying system was used, which integrates a sub-bottom profiler (SBP) and the Shipborne Over- and Under-Water Integrated Mobile Mapping System (SiOUMMS) on the same ship. The data acquired in this system include acoustic profile images and the over- and under-water topography of the pipeline route area. We also designed a position deviation correction method to improve the accuracy of the pipeline detection positioning, i.e., pipeline positioning correction in the real-time kinematic (RTK) positioning data and pipeline horizontal route correction in the integrated data. Compared with the uncorrected pipeline detection positioning result, the reliability of the pipeline inspection result is greatly improved, and the effectiveness and merit of the proposed method are clearly demonstrated. Finally, we conducted a buried pipeline safety assessment for the installation of newly designed wharf piles at Mawan Port of Shenzhen, China, where the results showed that one of the first rows of wharf piles would collide with the sewage pipeline.

Reliability functions for buried submarine pipelines in clay subjected to upheaval buckling

This paper presents the reliability analyses for studying and quantifying the variation of the reliability index (β) with the main parameters involved during the upheaval buckling of submarine buried pipes caused by high pressure and temperature conditions (HPHT). The analyses correspond to typical geotechnical and operational conditions of pipelines installed in the Bay of Campeche, offshore Mexico. The parameters that mainly control the vertical uplift capacity of pipe-soil system and considered as uncertain are: the undrained shear strength of soils (Su) and the soil cover height (H). Driving vertical loading depends mainly on the levels on temperature and pressure combined with vertical imperfections (δ), therefore those parameters were also considered as uncertain. Model errors for both capacity and driving vertical force were estimated and applied in the analyses.The limit state function was established by comparing the uplift pipe-soil capacity with the vertical driving loading. Parameter uncertainty was covered based on literature reviews, information of the proper conditions for pipelines operating in the Bay of Campeche and numerical analyses.A lower-bound capacity of the pipe-clayey soil system was included in the reliability analysis and is a novelty of this work. Lower-bound capacity is the smallest possible physical limit of the uplift pipe-soil capacity and its effect on the reliability is studied. Furthermore, finite element analyses were executed to evaluate the uplift capacity of the pipe-soil system and determine the analytical model error for estimating the capacity.The reliability indices (β) of the pipe-soil system under upheaval phenomena were evaluated by means of directional simulations. Consequently, the reliability functions β-δ and β-H/D (ratio cover height over pipe diameter) were quantified. It was found that the inclusion of the lower-bound capacity by means of a left-censored distribution possibly increases β in comparison to the estimations using conventional distributions, the maximum increments observed are in the range of 22–34%.

Engineering measures for preventing upheaval buckling of buried submarine pipelines

In-service hydrocarbons must be transported at high temperature and high pressure to ease the flow and to prevent the solidification of the wax fraction. The high temperature and high pressure will induce the additional stress in the pipeline, which results in the upheaval buckling of the pipeline. If such expansion is resisted, e.g., by the frictional effects of the foundation soil over a kilometer or of a pipeline, the compressive axial stress will be set up in the pipe-wall. When the stress exceeds the constraint of the foundation soil on the pipeline, suddenly-deforming will occur to release the internal stress, similar to the sudden deformation of the strut due to stability problems. The upheaval buckling may jeopardize the structural integrity of the pipeline. Therefore, effective engineering measures against this phenomenon play an important role in the submarine pipeline design. In terms of the pipeline installation and protection measures commonly used in Bohai Gulf, three engineering measures are investigated in great details. An analytical method is introduced and developed to consider the protection effect of the anti-upheaval buckling of the pipeline. The analysis results show that the amplitude of the initial imperfection has a great effect on the pipeline thermal upheaval buckling. Both trenching and burial and discrete dumping are effective techniques in preventing the pipeline from buckling. The initial imperfection and operation conditions of the pipelines determine the covered depth and the number of layers of the protection measures.

Discussion: Uplift resistance of buried submarine pipelines: comparison between centrifuge modelling and full-scale tests

A buried pipeline can buckle upwards if there is not enough resistance to movement. Both the uplift resistance (the maximum force per unit length) and the mobilisaion distance (the distance the pipeline has move before the maximum force is reached) are important in design. Tests on pipes in several soils were carried out at 1 g at full scale and in a centrifuge at 10 g and 1/10 scale. A comparison shows that the measured uplift resistances are consistent with the conventional understanding of centrifuge tests, but that the measured mobilisation distances are not consistent with that understanding. Unscaled mobilisation distance in the centrifuge is roughly the same as at full scale. Mobilisation distance does not scale with particle size. This discrepancy is related to the formation of shear zones, and to theory and observations in other areas of the mechanics of brittle materials, among them ice, concrete and heavily overconsolidated clays. An idealised analytical model displays the same behaviour.

Uplift resistance of buried submarine pipelines: comparison between centrifuge modelling and full-scale tests

Uplift resistance of buried submarine pipelines: comparison between centrifuge modelling and full-scale tests

Uplift resistance of buried submarine pipelines: comparison between centrifuge modelling and full-scale tests

A buried pipeline can buckle upwards if there is not enough resistance to movement. Both the uplift resistance (the maximum force per unit length) and the mobilisaion distance (the distance the pipeline has move before the maximum force is reached) are important in design. Tests on pipes in several soils were carried out at 1 g at full scale and in a centrifuge at 10 g and 1/10 scale. A comparison shows that the measured uplift resistances are consistent with the conventional understanding of centrifuge tests, but that the measured mobilisation distances are not consistent with that understanding. Unscaled mobilisation distance in the centrifuge is roughly the same as at full scale. Mobilisation distance does not scale with particle size. This discrepancy is related to the formation of shear zones, and to theory and observations in other areas of the mechanics of brittle materials, among them ice, concrete and heavily overconsolidated clays. An idealised analytical model displays the same behaviour.

Uplift resistance of buried submarine pipelines: comparison between centrifuge modelling and full-scale tests

A buried pipeline can buckle upwards if there is not enough resistance to movement. Both the uplift resistance (the maximum force per unit length) and the mobilisaion distance (the distance the pipeline has move before the maximum force is reached) are important in design. Tests on pipes in several soils were carried out at 1 g at full scale and in a centrifuge at 10 g and 1/10 scale. A comparison shows that the measured uplift resistances are consistent with the conventional understanding of centrifuge tests, but that the measured mobilisation distances are not consistent with that understanding. Unscaled mobilisation distance in the centrifuge is roughly the same as at full scale. Mobilisation distance does not scale with particle size. This discrepancy is related to the formation of shear zones, and to theory and observations in other areas of the mechanics of brittle materials, among them ice, concrete and heavily overconsolidated clays. An idealised analytical model displays the same behaviour.

FLOATATION MECHANISM OF A BURIED SUBMARINE PIPELINE UNDER CYCLIC LOADING OF WATER PRESSURE

FLOATATION MECHANISM OF A BURIED SUBMARINE PIPELINE UNDER CYCLIC LOADING OF WATER PRESSURE

Wave-induced uplift force on a submarine pipeline buried in a compressible seabed

A two-dimensional finite-element simulation of the wave-induced hydrodynamic uplift force acting on a submarine pipeline buried in sandy seabed sediments subject to continuous loading of sinusoidal surface waves is presented. Neglecting inertia forces, a linear-elastic stress-strain relationship for the soil and Darcy's law for the flow of pore fluid are assumed. The model takes into account the compressibility of both components (i.e., pore fluid and soil skeleton) of the two-phase medium. The results of numerical analysis are presented and discussed with respect to soil and pore fluid parameters where special attention is paid to the question of soil saturation conditions. The meaning of the results is also related to surface wave conditions. As a general conclusion, the practical, engineering recommendation is given in order to make a realistic, safe and economic estimation of the wave-induced uplift force acting on a buried submarine pipeline.